1. Field of the Invention
The present invention relates to puffer type circuit breakers, and in particular to such circuit breakers having a gaseous arc-extinguishing medium and which generate a blast of arc-extinguishing gas towards the contact area of the circuit breaker.
2. Prior Art
In circuit breakers of the so-called puffer-type, the gas blast energy is generated by compression of gas in a cylinder during the breaking operation. In order for such a breaker to manage a given interruption involving a certain current at a certain recovery voltage, the contact distance and the blast pressure have to exceed certain minimum values at one of the zero passages of the current. Since the phase position of the current may vary arbitrarily, such conditions have to be fulfilled for a sufficient length of time, for example 15 ms, so that a zero current passage occurs with certainty during that interval.
In conventional puffer-type circuit breakers, the pump piston is fixedly mounted and the cylinder is joined to the movable contact and moves together therewith. Such a design places heavy demands on the operating device of the circuit breaker, which operating device must have an accurately adapted motion and force characteristic in view of the above-mentioned demand on the blast pressure. It is particularly difficult to maintain the blast pressure above a certain minimum level for a sufficient time when a short breaking time is desired, for example for a two-cycle breaker, as the breaker must then be constructed with extra high contact speed, which, in an embodiment with a fixed pump piston, means that the pump motion is terminated within a relatively short time.
The puffer-type circuit breakers with a resilient piston which have been proposed previously also suffer from the above-mentioned drawbacks. In some such known designs, the spring of the piston of the pump means is compressed at the end of the opening operation of the circuit breaker through mechanical action from the operating device of the breaker, for example as described in German Offenlegungsschrift No. 1,765,153 and Swedish Pat. No. 369,352. In these designs the piston has no pressure-equalizing function but is arranged for the purpose of achieving a greater insulating distance between the contacts when the breaker is open, and a slowing-down of the movable contact and the puffer cylinder connected thereto.
According to other known proposals, a spring-loaded auxiliary piston is arranged in addition to the main piston for the purpose of achieving a quicker pressure increase in the puffer cylinder during the starting stage of the breaking operation. In a proposed design of this kind as described in British Pat. No. 1,438,590, the movable puffer cylinder comprises two coaxial cylinder spaces with different diameters, of which the cylinder space with the smaller diameter accommodates the fixed main piston, whereas the cylinder space with the larger diameter accommodates the auxiliary piston. The gas in the cylinder space of the auxiliary piston is pressed over into the main cylinder space during the starting stage of the breaking operation. During the critical final stage of the breaking operation, however, the auxiliary piston is inactive.
In another known design as described in British Pat. No. 1,438,708, the spring-loaded auxiliary piston is arranged in the puffer cylinder between the main piston and the contact area. The spring that affects the piston is dimensioned such that it is compressed at an early stage of the opening operation of the breaker. The auxiliary piston has a through opening provided with a nonreturn valve, enabling gas to flow in a direction from the space between the main piston and the auxiliary piston to the contact area. Because of the flow resistance at the opening, however, a rapid supply of extinguishing gas from that space is prevented when the pressure at the contact area is reduced during the interval which is critical for the breaking operation at the zero passage of the current.